This commit fixes a case where the peer blocks the shutdown process.
During the shutdown, the server will call `s.DisconnectPeer`, which
calls `peer.Disconnect`. Because the peer never enters `peer.Start` via
`s.peerInitializer`, the `startReady` chan will not be closed, causing
`peer.Disconnect` to hang forever. We now fix it by only block on
`startReady` when the peer is started.